1. Field of the Invention
This invention relates to integrated circuit testing, and more particularly to a nonintrusive noncontact dynamic testing technique using a pulsed laser to excite electron emissions as a function of dynamic operation of the circuit and to detect those emissions by an adjacent detector without the need for ohmic contacts or special circuitry on the integrated circuit chip or wafer.
2. Description of the Prior Art
A common way of testing an electronic circuit is a dynamic view of the circuit, voltage or currents, in operation, by means of an oscilloscope. The engineer probes the circuit at various test probe positions, for example by pricking certain landing pads with the points of test probes to make good contact, and views the resulting electrical conditions via analogous traces on the oscilloscope viewing tube. As circuits have become more compact and complex, it has become more and more difficult to accomplish nonintrusive positioning of test probe points without the danger of destroying the circuit, or at least of providing such a relatively enormous change due to the physical mass of the probes that the test becomes suspect. A totally nonintrusive contactless oscilloscope is a known desire. One approach to the contactless oscilloscope is the e-beam tester, but the e-beam is somewhat intrusive in that the high energy (100 electron volts or more) of the incident electrons may cause permanent material damage where the e-beam strikes the surface being tested.
U.S. Pat. No. 3,129,353, Nemes, "Multiple Radiation Source Microscope," Apr. 14, 1964, shows several accelerating energy electrodes in tandem, controlled as needed to apply a given amount of radiation energy to a test specimen in a plurality of summed wavelengths for examining the internal and external structure of a particular specimen in a microscope. Nemes does not make any attempt to measure voltage.
U.S. Pat. No. 3,370,168, Komoda, "Anode Aperture Plate For a Television Camera Tube in an Electron Microscope Comprising a Stainless Steel Foil," Feb. 20, 1968, shows an aperture plate for an electron microscope. The aperture plate is made of a thin highly light transmissive stainless steel layer in order to limit obstruction of the radiation travelling through the plate. Komoda is concerned with the electron bombardment induced conductivity. Komoda scans an induced-conductivity film with a low velocity scanning electron beam from an electron gun and uses the gain of the induced conductivity effect to control a fluorescent plate for providing the picture for television scanning. Komoda does provide an anode aperture plate made of stainless steel foil whose aperture part is heated by bombardment of a scanning electron beam. Komoda is not concerned with measuring voltages but rather is concerned with developing a television camera tube for installation in an electron microscope. The following are systems representative of the prior art: U.S. Pat. No. 4,266,138, Nelson et al, "Diamond Targets For Producing High Intensity Soft X-Rays & a Method of Exposing X-Ray Resists," May 5, 1981. Nelson shows a technique for exposing x-ray sensitive resists to carbon K x-rays using a type 2B diamond target which dissipates considerably more power and produces higher intensity x-rays than graphite targets. This is a production technique, not a testing technique.
U.S. Pat. No. 4,417,948, M. Baton, "Self-Developing Photoetching of Polyesters by Far UV Radiation," Nov. 29, 1983, describes a technique for photoetching polyesters by application of ultraviolet radiation in the presence of oxygen. This is also a production technique, not a testing technique.
U.S. Pat. No. 4,380,864, P. K. Das, "Method for Providing In-Situ Non-Destructive Monitoring of Semiconductors During Laser Annealing Process," Apr. 26, 1983. Das describes a technique for positioning a surface acoustic wave device adjacent to a semiconductor being annealed. Affixing an electrical contact to the top surface of the semiconductor and using a composite of the transverse surface acoustic wave and the charge carriers of the semiconductor to produce a transverse acoustal electric voltage which thus is a function of the semiconductor conductivity. This is a contact technique, not a contactless technique.
U.S. Pat. No. 4,332,833, Aspnes et al "Method for Optical Monitoring in Materials Fabrication," June 1, 1982. Aspnes et al, shows a technique for utilizing the sensitivity of the dielectric function of a crystal to crystalline volume fractions and recognizing that the volume fractions vary as a function of the measured dielectric function over an appropriate range of frequencies corresponding to photon energies of approximately 1.5 electron volts to 6 electron volts to do a dynamic monitoring of deposition within a reactor. This is a contactless optical technique for monitoring materials during thin film processing; it is not a current-voltage tester.
U.S. Pat. No. 4,408,883, Iwamoto et al, "Apparatus for Inspecting Average Size of Fundamental Patterns," Oct. 11, 1983. Iwamoto et al shows an apparatus for determining the average size of fundamental patterns by comparing Fourier transforms of pattern images in a processor and providing the patterns by applying coherent light focused on a target for test and monitoring the reflected images by converting them to Fourier transforms and comparing the Fourier transforms against known image patterns. This is a contactless optical technique for image recognition; it is not a current-voltage tester.
Fazekas, Feuerbaum and Wolfgang, "Scanning electron beam probes VLSI chips," Electronics Magazine, Vol. 54, No. 14, July 14, 1981, shows an electron beam probe for testing electronic chips. The Fazekas et al article shows a system for testing integrated circuits with an electron beam which for loadless probing they suggest a primary electron energy of 2-3 kiloelectronvolts to achieve a charge balance in which when one electron strikes the integrated circuit, another leaves it again. This permits voltage contrast in which secondary electrons emitted at a +5 volt metallic land are repelled by local electric fields while those from a ground pad are accelerated to a collector. Positive interconnections show up as dark in a resulting image, while negative ones show up as light. The E-beam is used to select a particular area for probing. Fazekas et al also shows how to do actual voltage measurements by using a secondary electron spectrometer added to the sampling electron microscope. The spectrometer extracts secondary electrons emitted by the integrated circuit, slows the electrons in a retarding field and then deflects them to the collector.
J. E. Carroll and J. K. A. Everard, Proceedings of the 9th European Microwave Conference, Microwave 79, Brighton, England, pages 543-547, 17-20, 1979, shows a technique for using a pulsed laser as a light radiation source, with frequency and energy level being adjusted by tandem harmonic generators. The Carroll et al article splits the laser pulse in order to get different laser pulse wavelengths to cause penetration to different levels in a Trapatt diode under study. Carroll et al thus shows that a laser light can be split into red and green fractions for use in sampling a diode under test. Carroll et al does not use the laser light to activate photoemission of a sample but rather uses the laser simply as a timing device for varying time of injection of optically generated charge.
Rubloff, "Contactless Measurement of Voltage Levels Using Photoemission," IBM Technical Disclosure Bulletin, Vol. 25, No. 3A, August 1982, pp. 1171-1172, shows ultraviolet light stimulation of photoemission for contactless measurement of voltage levels, but not in the context of dynamic testing, and not with a laser (c.w., or pulsed). The author is a coinventor. Henley, "Logic Failure Analysis of CMOS Using a Laser Probe," Spectrum Sciences, 3050 Oakmead Village Drive, Santa Clara, Calif., 95051, shows a laser contactless probe for an integrated circuit, using available pin connections to conduct stimulated current-voltage signals to a computer for analysis. Henley does not use an adjacent detector, but must dedicate a certain amount of circuitry (external to the integrated circuit under test) to the conduction of test signals. No photoemission from the circuit is created. The logic state of the integrated circuit is determined by interrupting its dynamic operation and then measuring the current transient induced in the power supply by laser light absorption in an active semiconductor region (the light does not impinge on the metal wires and nodes of the circuit); as a result, neither logic states nor AC switching waveforms are determined during dynamic operation of the circuit.
The prior art does not teach nor suggest the invention, the technique of stimulating electron emission signals from in an integrated circuit by the use of a pulsed laser for accurate time-resolution, without actual contact, by focussing a modified laser beam on a portion of the integrated circuit while in operation with signal dynamics, and monitoring those emissions for test against appropriate norms, to determine whether the integrated circuit is operating properly.